How to extrude vertex normals along an angle with an even offset?
You can use simple extrude and scale, however along custom Transform Orientation, in this case View, in order to lock scaling on non-desired axes.

Select the face which is oriented at an angle (which looks like parallelogram). In the case of cuboid on your screenshot the one which is formed with the top loopcut on the side.  

Align viewport with it so you see the face from the top. Shift+Numpad 7 will do that.  

Once viewport is aligned, select the ring of faces to extrude (Alt + Left click on faces near an edge).
Make View as transform orientation along which transform will be done - press Alt+Space and choose View.

Press E to extrude, cancel default extrusion and press S to execute Scale tool.
Now lock movement on axis you don't want to interfere. In case of described viewport position View X axis should be locked, so after enabling Scale tool press Shift+X (locks global X) and then again Shift+X (locks View X). Move to the desired offset.